Oxides are formed on steel every day, both intentionally and inadvertently. In the case of electrical steel, for example, semi-processed steel is punched into suitable shapes, and the resultant laminations are stacked under substantial pressure due to the weight of the laminations, and subjected to a final anneal. The purpose of the final anneal is to relieve stress and grow grains and/or to decarburize the steel if necessary. Such annealing typically results in some incidental oxide formation on the surface of the laminations. The oxides that are formed may include a mixture of oxides of iron, including magnetite, hematite and wustite. Typical annealing conditions that produce such incidental oxides do not result in a surface resistivity of the laminations above that of conventional steam bluing methods.
Oxide layers and coatings have been formed on or applied to electrical steel laminations or strip in an attempt to provide the laminations with electrically insulating characteristics. Under conditions of alternating magnetization, electrical units such as motors and transformers that are formed from the laminations, are subject to certain power losses. The component of the loss that is attributable to the core of the unit is known as core loss. One component of core loss is eddy current loss, which is reduced by forming the core with laminations rather than as a solid mass. The laminations must be sufficiently insulated from one another to effectively reduce eddy current loss. Users of semi-processed electrical steel often apply an insulating coating to the strip before lamination punching. However, this process is costly and the coatings may lose their insulating ability or experience adhesion loss after annealing.
An oxide layer of magnetite may be intentionally formed on the surfaces of the laminations during the final stage of the anneal by a process known as "bluing" in which the laminations are subjected to steam or air diluted with an inert gas, typically nitrogen. This approach forms a layer of predominantly magnetite on the laminations that typically has a surface resistivity characterized by a Franklin amp (F-amp) value above 0.90, where 1.0 F-amp is a dead short condition. A perfect insulator is characterized by an F-amp value of 0.0.
One atmosphere that is typically used during annealing is known as a DX atmosphere or EXOGAS, which is formed of partially combusted fuel gas. In particular, natural gas (i.e., methane) is burned in air, which lacks sufficient oxygen for complete combustion. The resultant DX gas produced by this incomplete combustion comprises the following gases: CO, CO.sub.2, H.sub.2, water vapor, N.sub.2, O.sub.2 and unburned methane. An HNX atmosphere is another atmosphere used in annealing, though less extensively than the DX atmosphere. The HNX atmosphere includes H.sub.2 and N.sub.2 gases in major amounts with optional added water vapor.
The amount of water vapor present in the annealing furnace atmosphere is described in terms of dew point--the temperature at which water vapor in the furnace condenses. In the DX atmosphere the presence of some amount of water vapor is virtually unavoidable. In contrast, in the HNX atmosphere water vapor may be intentionally added. Regardless of the amount of water vapor that is intended in the annealing furnace, some water vapor is inevitably present. Water vapor may be present because, in the case of decarburizing, a higher dew point is desirable, because the annealing furnace is old and no longer effectively sealed, or because seals are broken due to opening of doors and the like to admit and expel laminations into and from the furnace.
The DX atmosphere has traditionally been used to decarburize high carbon content steel. Decarburizing is enhanced by factors such as raising the dew point of the atmosphere and decreasing the amount of hydrogen gas in the case of the DX atmosphere. Although the DX atmosphere is still used today to anneal ultra low carbon content steels, it is not necessary since carbon does not need to be removed therefrom by decarburization and thus, a lower dew point could be used.
A challenge with the formation of oxide layers on electrical steel in laminated stacks is the degree of penetration of the oxide onto interior surfaces of the stack. Since the laminations are stacked under substantial pressure due to the weight of the laminations, the penetration of the oxidation atmosphere into the compressed areas of the stack is generally poor. Depending on the size and configuration of the stack, little or no oxide formation is typically observed on the back iron surfaces of the electrical steel. In contrast, the peripheral areas of the electrical steel in the laminated stack are readily exposed to the oxidizing atmosphere and have the greatest oxide thickness. As a result, the insulative properties are not uniform and bare or thin oxide areas may cause shorts or electrical sticking failures depending on the particular design of the electrical units formed from the laminations.